A British passenger jet flying into Birmingham with five crew and 42 passengers on board narrowly avoided disaster after fumes leaked into the cabin and overcame the pilots.

Official documents, seen by The Observer, show that the pilot and co-pilot almost passed out after fumes, thought to have come from the engines, filled the cabin. The incident, last November, is the latest example of what critics say is a serious safety risk in the British-built BAe 146.

The British-European aircraft was flying into Birmingham from Paris on 5 November last year. Shortly after take-off passengers reported an oily smell in the rear of the cabin. Just before the aircraft began its descent into Birmingham International Airport, the captain, followed by the co-pilot, left the flight deck to go to the toilet. After returning to the cockpit, they both reported feeling ill.

The captain said he felt nauseous, had difficulty concentrating on the flight path and told the crew that his vision had become blurred. The co-pilot's pupils dilated sharply and he needed two stewards to help him put on his oxygen mask. His hands were trembling and he could not speak normally.

The captain managed to land the plane safely. He and the co-pilot were taken to hospital on arrival. The incident is being investigated by the Air Accidents Investigation Branch at Farnborough.

Some senior aviation experts suggest the cause of the incident was a broken or damaged engine oil seal, which allowed engine fumes to leak into the cabin through the air-conditioning system.

The danger of fumes leaking into the cabin of the BAe 146 was officially acknowledged last year after it was exposed by The Observer. Leaked documents revealed that there have been dozens of suspected leaks, putting at risk thousands of holiday-makers.

On one flight to Sweden toxic fumes made the captain 'dizzy and groggy'. He was forced to use an oxygen mask to continue flying the plane but the symptoms persisted and he prepared to declare an emergency. Many passengers complained that they felt 'like zombies' and could not stay awake. The first officer eventually managed to land the plane safely. Some passengers could not be woken even after landing.

The incident was described by Swedish air safety authorities as 'extremely serious'. They reportedly found that faulty seals allowed engine oil, containing organophosphates, to pollute the cabin.

Following The Observer's revelations last year, the CAA issued an Airworthiness Directive (AD) ordering airlines to investigate the problem. Many major airlines use the plane and the royal family has three.

Some aviation experts believe the jet may have a fundamental design flaw. Chris Winder, head of safety sciences at Australia's University of New South Wales, who has studied the 146, said: 'We call it the Flying Gas Chamber. I can't understand why the authorities let it keep flying. It's an accident looking for a place to happen.'

BAE Systems, the manufacturer, stresses the aircraft is internationally certified as safe by all the major aviation authorities, including the FAA in the United States and the CAA in Britain.

Mike Peters, a spokesman for the company, said he was aware that an incident on 5 November last year but could not discuss it before the investigation had been completed. 'To speculate now on what the cause might have been is totally wrong. The quality of cabin air is an issue across the whole airline industry and BAE Systems has co-operated fully with all investigations into the issue.'